1. Field of the Invention
The present invention relates in general to a method for fabricating semiconductor devices. More particularly, the present invention relates to a method for removing contamination that contains at least one element belonging to one of 3A group, 3B group and 4B group of long-period form of periodic system of elements.
2. Description of the Related Art
The miniaturization and large-scale integration required in development of MOSFETs have accelerated extensive study and effort on reducing thickness of gate insulators. The reduction of gate insulators to several nanometers thick has been accomplished at the present level of technology. However, further reduction in thickness provides poor device performance due to its gate leakage current.
To go beyond this thickness barrier, the high-k films like refractory metal oxides ZrOx, HfOx and AlOx are expected to play an important role as gate insulator materials although silicon dioxide has thus far proved to be the perfect gate insulator.
Tantalum oxide TaOx is now under study as capacitor material of DRAMs thanks to its high dielectric property. However, the occurrence of leakage current poses problem to be solved. Again, the high-k films like zirconium oxide ZrOx and hafnium oxide HfOx are expected to play an important role as capacitor materials.
However, the oxides of 3A-group, 3B-group and 4A-group elements are difficult to remove. If the contamination of such material as above is adhered to the surface of a silicon substrate or to a film of silicon oxide, it is very difficult to remove because the element(s) belonging to 3A-group, 3B-group and 4A-group react with silicon material to form silicides or silicates.
Hydrofluoric acid is known as a strong remover liquid for the contamination on the surface of silicon substrate. Hydrofluoric acid can remove the contamination due to lift off process involving dissolution of a portion of the silicon substrate. The technique of this kind is disclosed in JP-A 5-21595 and JP-A 10-307087.
However, hydrofluoric acid alone has proved to be insufficient for removing contamination of high-k film. The films of different materials were etched with an aqueous solution of hydrofluoric acid by 50 mass %. The results are
Thermal silicon oxide film: 1000 nm/minute;                ZrO2 film: 2 nm/minute;        HfO2 film: 0.5 nm/minute.        
The above list clearly indicates that the high-k material is difficult to remove due to it low etch rate.
The present invention aims at providing a technique to remove sufficiently metal(s) formed on a semiconductor substrate, the metal(s) belonging to one of 3A group, 3B group and and 4B group of long-period form of periodic system of elements.